The present invention relates generally to nickel-cadmium batteries, and in particular, to a sealed nickel-cadmium battery comprising porous electrodes of opposite polarity with a porous separator between them.
Sealed nickel-cadmium batteries cannot be operated without running the risk of electrochemical reduction of oxygen developing during overcharge. However, auxiliary electrodes can be used as a means for controlling this reduction. Moreover, since these auxiliary electrodes are often provided with an independent current lead, they can also be used to detect oxygen and hydrogen, or to assume the function of a reference electrode, for example, to provide a signal indicating that full charge has been reached when a preset potential threshold is exceeded.
Auxiliary electrodes serving as oxygen-consuming electrodes are preferably porous in order to present the largest possible surface for oxygen reduction. To set the potential necessary for this, it is sufficient to electrically short-circuit such electrodes with the negative electrode of the battery.
Presently available auxiliary oxygen electrodes generally consist of a practically inert metal, having a lattice-shaped or net-shaped design, or a porous fabric or plastic structure into which suitable catalysts are embedded. The surface of the resulting electrode is preferably positioned immediately adjacent to the negative electrode of the battery. As is known from DE-OS 28 26 780, if desired, the auxiliary electrode can be provided with a hydrophilic layer facing toward the principal electrode, for catalyzing the oxygen reduction, and a hydrophobic layer facing away from the principal electrode, according to the principle of an air cathode. In view of the fact that oxygen reduction must take place at the gas-electrolyte-solid three-phase boundary, the catalyst material, which usually contains carbon or silver, is capable of being arranged in the gas space, or at least near the gas space, at least in cells with free electrolyte.
It has been found that in sealed nickel-cadmium sintered cells (round cells), when the cell is carefully balanced (i.e., the charging and discharging reserves are correct), the sintered nickel skeleton which is exposed in some areas on the edges of the cadmium electrode so strongly influences the oxygen turnover that such cells can be operated safe from overcharge even without additional measures to support any oxygen consumption. However, a marked impairment of the oxygen-consumption mechanism is presented when the negative sintered electrodes are replaced by cadmium electrodes of a different structure, such as the so-called electro-deposit type.
Electrodes of this type are manufactured according to a special process such as is described in DE-OS 28 22 821, wherein a perforated nickel strip is first coated with cadmium, in an electrolyte bath within which is dissolved a cadmium salt; and thereafter passed between calender rollers located outside of the electrolyte trough, so that the initially powdery and poorly adhering active metal layer is strengthened. Such cadmium electrodes, as well as other pressed cadmium electrodes, have certain disadvantages as compared with sintered electrodes concerning their ability to withstand rapid current-consumption.
It is therefore the object of the present invention to improve the chargeability of batteries of this type, with high currents.